Piston rings act as bridges for the transmission of heat between the piston and the cylinder liner, from which it is dissipated through fins or by transfer to water present in cooling channels.
The heat originating from explosion of the air-fuel mixture causes the piston ring to expand. As the ring acts as a “bridge” conducting heat between the piston and the block, the heat (and as a consequence the temperature) is greatest in the inner portion of the ring, so that it expands more than the outer portion. This difference in expansion is greater the greater the thermal gradient. The different expansion along the radial line of the ring has the effect that the radial pressure exerted by its free ends (tips) in contact with the cylinder wall increases excessively, undesirably increasing the rate of wear of the ring at that point.
Wear at the free ends has an adverse effect on the performance of the ring as regards sealing of the combustion chamber and/or control of the oil film and makes it necessary to develop techniques which maintain this at an acceptable level. In this respect, contrary to the present invention, the great majority of alternative solutions up to the present time have relied on an old paradigm according to which wear at the free ends was treated by the application of more highly developed coatings and/or by adjusting the geometrical format of the ring in such a way as to considerably reduce the radial pressure exerted by the free ends of the ring.
Japanese Patent Document JP 2008-057671 discloses a piston ring provided with inner peripheral portions made of a material having a lower thermal expansion coefficient than the material forming other portions of the ring. In this way the radial pressure exerted by the different portions of the ring is harmonized independently of the operating condition of the engine.
German Patent Document DE 10 2007 029992 relates to a piston ring in which the free ends comprise a material whose thermal expansion coefficient is lower than that of the rest of the ring, ensuring that the expansion, and therefore the radial pressure, increases less in this region. As a result of this, wear at the free ends is less than in comparison with a conventional ring.
Japanese Patent Document JP 2008-069884 relates to a combined piston ring for control of the oil film which ensures good performance at any operating temperature. For this the ring comprises a cooling element C positioned within the interior of the expanding element, which absorbs heat and therefore reduces the resulting radial deformation of the ring, in particular at the free ends.
Patent Document WO 2008/016010 relates to a piston ring to which the applied stress varies between low temperature low load conditions and high load and temperature conditions in such a way as to minimize friction losses and increase engine performance (less fuel consumption). To achieve this the ring is made of a titanium-tantalum shape-memory alloy comprising tantalum in a proportion of not less than 30% in moles and less than 40% in moles, the remainder being titanium and the inevitable impurities.
Patent Document WO 2008/016009 relates to a piston ring in which the applied stress varies between low temperature low load conditions and high load and temperature conditions in such a way as to minimize friction losses and increase engine efficiency (less fuel consumption). According to the document this is achieved through a shape memory alloy comprising 34.7 to 48.5% in moles of nickel, 9 to 22.5% in moles of either of the elements zirconium and hafnium and 1 to 30% in moles of niobium, the remainder being titanium and the inevitable impurities.
Patent Document WO 2009/069703 relates to a combination of a piston ring and a cylinder liner which satisfies a plurality of hardness and surface finish conditions so that linear expansion of the ring matches the linear expansion of the cylinder, thus ensuring the durability of the whole.
Patent Document WO 2009/069762 relates to a steel for the manufacture of piston rings and a piston ring manufactured therewith which ensures a thermal expansion coefficient that is very similar to that of an aluminum cylinder liner, making it possible to use this ring in aluminum liners with excellent results as far as sealing is concerned. The product (ring) comprises (by mass) 0.01% to 1.9% of carbon, 0.01% to 1.9% of silicon and 5.0% to 24.0% of manganese, the remainder being iron and the inevitable impurities. In addition to this the ring comprises 18.0% or less of chromium and/or 12.0% or less of nickel in addition to the essential elements above. In addition to this the product may contain 1% or less of aluminum and/or 0.3% or less of nitrogen, as well as one or more of the elements from niobium, titanium, zirconium, molybdenum and copper in a percentage of up to 4.0%.
German Patent Document DE 10 2007 035502 relates to a piston ring having an anti-wear coating formed of a material having a thermal conductivity of at least 180 W/(m·K), in which at least 5% by volume of the coating layer comprises an aluminum nitride.
Japanese Patent Document JP 4008327 relates to a piston ring comprising an outer metal ring on top of a resin ring. The resin ring acts to reduce/control thermal expansion of the ring, maintaining it at an acceptable value from cold starting of the engine up to operation at high rotation speeds.
Japanese Patent Document JP 2004-340075 relates to a piston ring of the oil scraper type in two parts which comprises two expansion rings, each formed of a specific material, one of shape-memory alloy with a thermal expansion coefficient which is different from the other one. In this way control of the percentage of thermal expansion of the ring is achieved in different motor operating situations.
Japanese Patent Document JP 2003-042294 relates to a piston ring to which a hard carbon coating is applied, and this, in addition to acting as an abrasion-resistant film, has a thermal expansion coefficient which is greater than or equal to [15×10−6/° C.]. In this way suitable behavior of the ring as regards thermal expansion is achieved.
Japanese Patent Document JP 2004-156472 comprises an oil ring in three parts provided with a spacer comprising two members welded together, in which the second member comprises a material having a thermal expansion coefficient greater than that of the material comprising the first member. In this way the resulting ring has good performance in different engine operating situations.
Finally, American Patent Document US 2011/0312860 relates to a coating for covering components subject to wear and high operating temperatures, such as parts of gas turbines and piston rings. In addition to resisting operating abrasion this coating also acts as a thermal barrier to minimize loss of performance in high temperature operating situations.
Describing it in a generic manner, the coating disclosed in US 2011/0312860 comprises a hard ceramic phase, a metal phase having an adhesive function and a lubricating phase comprising a multi-component oxide. Nevertheless, in the description of this document there is no discussion about the properties of the coating with regard to reducing the thermal expansion occurring in the mechanical component to which it has been applied.
As will be seen, all the documents in the state of the art above, which are very representative of the state of the art for piston rings in internal combustion engines, are based on an earlier paradigm in which the material of the part is altered so that thermal expansion values lie within desired parameters, and none of them clearly discuss use of a thermal coating which acts to a greater or lesser extent as a thermal barrier.
No piston ring has yet been developed to reduce radial expansion of the ring, mainly in the region of the tips, thereby increasing the service life of the ring and minimizing the need to apply specific coatings which are more resistant to wear at the contact surface with the cylinder, through the application of an additional thermal coating (in addition to the anti-wear coating present in this type of component) in places where there is transfer of heat from the gases or the hottest parts of the piston to the ring (such as for example the contact faces between the ring and the piston).